1. Field of the Invention
The present invention relates to a high-tension steel plate which has a tensile strength of 1100 MPa or more, and is excellent in base metal toughness and heat-affected zone (HAZ) toughness. The high-tension steel plate of the invention is used suitable as a thick steel plate used for construction machinery, industrial machinery and others.
2. Description of Related Art
A thick steel plate used for construction machinery, industrial machinery and others has been required to have a higher-strength performance with a recent increase of needs that steel plates should be made lighter. The thick steel plate used for these products is also required to have a high toughness (each of base metal toughness and HAZ toughness). In general, however, the strength and the toughness tend to be conflicting with each other. As the strength becomes higher, the toughness becomes lower.
For example, JP 2009-242832 A describes a technique of a high-tension steel plate which is excellent in bending workability while maintaining a high strength that is a tensile strength (TS) of 980 MPa or more. This prior art attains an expected object by using a component system to which none of elements high in solid-solution strengthening power, such as Cu and Ni, which have been hitherto added to make steel higher in strength, are added, and further adding respective appropriate amounts of Ti and Nb thereto, thereby making the prior austenite (γ) grain diameter finer.
However, according to this prior art, components in the steel are not appropriately controlled, so that the steel cannot ensure a high HAZ toughness. According to the prior art, Ti is added thereto to control the microstructure. However, the present inventors' investigations have demonstrated that the steel is deteriorated in base metal toughness, in the range of high strengths of 980 MPa or more, by effect of a Ti inclusion.
Furthermore, a thick steel plate used for construction machinery, industrial machinery and others is preferably required to have an excellent abrasion resistance besides high strength and toughness. Generally, the abrasion resistance and the hardness of a thick steel plate are correlative to each other. Thus, a thick steel plate concerned for being abraded needs to be made high in hardness. In order to ensure a stable abrasion resistance, the thick steel plate needs to have a hardness even over its regions from its surface to its plate-thickness-direction internal portion (near t/2 wherein t is the thickness) (i.e., to be similar in hardness between the surface and the internal portion of the thick steel plate).